Quadrature phase-shifting networks are widely used in electronic systems. Very often, the overall bandwidth of the electronic system will depend upon the bandwidth of the quadrature phase-shifting network.
One well-known type of quadrature phase-shifting network is the coupled transmission line, which includes two quarter-wavelength unbalanced transmission lines, the “center” conductors of which are placed in physical proximity so as to provide coupling. Such coupled transmission-line phase shifters have a bandwidth in the range of ten percent.
Many modern electronic systems are desirably implemented in the form of monolithic integrated circuits. As such, the phase shifters must provide the desired phase shift while defined on, or as part of, a monolithic integrated circuit. Such integrated circuits may have substrates made from semiconductor material, such as silicon. Semiconductor, when used as a substrate for passive electronic components, such as capacitors or inductors, tends to introduce attenuation or ohmic losses, which undesirably affect the operation.
There are two other well-known types of quadrature phase shifter in addition to the coupled-transmission-line quadrature phase shifters. These include the all-pass filter and the polyphase network. FIG. 1 illustrates a prototype or single stage of the all-pass phase-shifting network 10. In FIG. 1, an all-pass phase shifting network 10 includes a balanced two-conductor “input” port 12 with two nodes or terminals 121 and 122. A pair 16 of non-coupled inductors includes inductor elements 161 and 162. Inductor element 161 defines first and second terminals 1611 and 1612. Inductor element 162 defines first and second terminals 1621 and 1622. Terminal 1611 of inductor element 161 is connected to terminal 121, and terminal 1622 of inductor element 162 is connected to terminal 122. All-pass phase shifting network 10 of FIG. 1 also includes a further pair of balanced “output” ports 141 and 142 of a set 14 of output ports. Output port 141 defines I+ and I− terminals 1411 and 1412, respectively, and output port 142 defines Q− and Q+ terminals 1421 and 1422, respectively. Terminal 1612 of inductor element 161 is connected by way of a conductive path 202 to terminal 1422, and terminal 1621 of inductor element 162 is connected by way of a conductive path 201 to terminal 1421. A capacitor 181 is connected “between” terminals 121 and 1411, and a capacitor 182 is connected between terminals 122 and 1412. A resistor 221 is connected “between” terminals 1411 and 1421, and a resistor 222 is connected between terminals 1412 and 1422.
The mutually 90° phase shifted signals appear at output ports 141 and 142. More particularly, with respect to FIG. 1, the output I+ at terminal 1411 represents zero (0) degree phase shift. The output I− at terminal 1412 represents −180 degree phase shift. The output Q+ at terminal 1422 represents −90 degree phase shift. And the output Q− at terminal 1422 represents −270 degree phase shift. Such a single stage of differential quadrature all-pass filter exhibits a bandwidth of about 2:1, and a through loss or attenuation of about five (5) dB. It should be noted that the loss of 5 dB is attributable to both ohmic or heat losses and to power division. That is, the applied power is divided among two output ports, so the power available at any given output port will in theory be only one-half of the applied power. This corresponds to a theoretical loss of 3 dB regardless of the efficiency of the circuit.
It should be noted that the terms “between,” “across,” and other terms such as “parallel” have meanings in an electrical context which differ from their meanings in the field of mechanics or in ordinary parlance. More particularly, the term “between” in the context of signal or electrical flow relating to two separate devices, apparatuses or entities does not relate to physical location, but instead refers to the identities of the source and destination of the flow. Thus, flow of signal “between” A and B refers to source and destination locations, and the flow itself may be by way of a path which is nowhere physically located between the locations of A and B. The term “between” can also define the end points of the electrical field extending “across” or to points of differing voltage or potential, and the electrical conductors making the connection need not necessarily lie physically between the terminals of the source. Similarly, the term “parallel” in an electrical context can mean, for digital signals, the simultaneous generation on separate signal or conductive paths of plural individual signals, which taken together constitute the entire signal. For the case of current, the term “parallel” means that the flow of a current is divided to flow in a plurality of separated conductors, all of which are physically connected together at disparate, spatially separated locations, so that the current travels from one such location to the other by plural paths, which need not be physically parallel.
In addition, discussions of circuits necessarily describe one element at a time, as language is understood in serial time. Consequently, a description of two interconnected elements may describe them as being in “series” or in “parallel,” which may be true for the two elements described. However, further description of the circuit may implicate other interconnected devices, which when connected to the first two devices may result in current flows which contradict the “series” or “parallel” description of the original two devices. This is an unfortunate result of the limitations of language, and all descriptions herein should be understood in that context.
Also, the term “coupled” as used herein includes electrical activity extending from one element to another element either by way of an intermediary element or in the absence of any intermediary element.
The terms “input” and “output” in the case of passive networks such as those of FIG. 1 are for ease of identification and are not necessarily descriptive of the use, as such networks are “reciprocal,” in that their actions are independent of the direction of energy flow therethrough.
FIG. 2A illustrates a prototype of a single-pole polyphase filter 210, and FIG. 2B illustrates a prototype of a double-pole polyphase filter 250. In FIG. 2A, single-pole polyphase filter 210 includes a balanced input port 212 including input + terminal 2121 and − terminal 2122, and output ports 2141 and 2142. Output port 2141 defines I+ terminal 21411 and I− terminal 21412, and output port 2142 defines Q+ terminal 21421 and Q− terminal 21422. A resistor 2221 is connected between terminals 2121 and 21411. A resistor 2222 is connected between terminals 2121 and 21421, a resistor 2223 is connected between terminals 2122 and 21412, and a resistor 2224 is connected between terminals 2122 and 21422. Also, a capacitor 2181 is connected between terminals 2121 and 21421, a capacitor 2182 is connected between terminals 2121 and 21412, a capacitor 2183 is connected between terminals 2122 and 21422, and a capacitor 2184 is connected between terminals 2122 and 21411.
The single-pole arrangement of FIG. 2A provides quadrature phase shift between ports 2141 and 2142, but with relatively limited bandwidth because of its single-stage nature. More particularly, with respect to FIG. 2A, the output I+ at terminal 21411 represents zero (0) degree phase shift. The output I− at terminal 21412 represents +180 degree phase shift. The output Q+ at terminal 21421 represents +90 degree phase shift. And the output Q− at terminal 21422 represents +270 degree phase shift.
The multistage filter 250 of FIG. 2B provides greater bandwidth than the single-stage filter 210 of FIG. 2A. Filter 250 of FIG. 2B includes a balanced input port 252 with + terminal 2521 and − terminal 2522, and balanced output ports 2541 and 2542. Output port 2541 includes I+ terminal 25411 and I− terminal 25412, and output port 2542 includes Q+ terminal 25421 and Q− terminal, 25422. A resistor 2721 of a set 272 of resistors extends, or is coupled, from terminal 2521 to a node 2801, a resistor 2722 extends from terminal 2521 to a node 2802, a resistor 2723 extends from terminal 2522 to a node 2803, and a resistor 2724 extends from terminal 2522 to a node 2804. A capacitor 2681 of a set 268 of capacitors is coupled between terminal 2521 and node 2802, a capacitor 2682 is coupled between terminal 2521 and node 2803, a capacitor 2683 is coupled between terminal 2522 and node 2804, and a capacitor 2684 is coupled between terminal 2522 and node 2801. Also in FIG. 2B, a resistor 2725 of set 272 of resistors is coupled from node 2801 to terminal 25411, a resistor 2726 is coupled from node 2802 to terminal 25421, a resistor 2727 is coupled from node 2803 to terminal 25412, and a resistor 2728 is coupled from node 2804 to terminal 25422. A capacitor 2685 of set 268 of capacitors is coupled between node 2801 and terminal 25421, a capacitor 2686 is coupled between node 2802 and terminal 25412, a capacitor 2687 is coupled between node 2803 and terminal 25422, and a capacitor 2688 is coupled between node 2804 and terminal 25411.
The bandwidth of filter 250 of FIG. 2B is about 3:1. However, the attenuation or through loss is about 12 to 15 decibels (dB). Quadrature filters are desired which provide relatively low through loss or attenuation and broad bandwidth.